The present invention relates to proteins and polypeptides that are capable of interaction with the human CD33 protein, and nucleic acids encoding the same. CD33 is a cell surface marker used to differentiate between acute lymphocytic and acute myelocytic leukemias. The regulation of CD33, including those elements to which it binds in vivo, is not fully understood, and there is still a need for investigation of this biological system. The present invention sets out to address this need.